GB-A-1,266,351 discloses a number of beverage containers where a secondary chamber is provided which contains gas charged to a pressure substantially above atmospheric pressure. In one example, the secondary chamber is permanently in communication with the container via a restricted orifice and is charged with gas under pressure at the time of filling of the container. In another example, the secondary chamber is filled with gas and the restricted orifice sealed with gelatine or other non-toxic substance which is intended to retain the gas under pressure within the secondary chamber prior to and during filling but which dissolves after contact with the beverage for a period of time to open the restricted orifice. In a further example, the restricted orifice is provided in a flexible wall of the chamber which is exposed to the pressure in the main body of the container, the arrangement being such that pressure in the main body of the container holds the region of the wall around the restricted orifice sealed against a grommet until the container is opened, whereupon the resultant release of pressure results in the seal being broken and permits the gas under pressure from the secondary chamber to jet into the beverage through the restricted orifice. For a variety of reasons, none of these designs have met with commercial success.
GB-A-2,183,592 discloses a beverage container wherein, instead of gas being jetted from the secondary chamber by way of a restricted orifice, carbonated beverage or carbonated beverage followed by gas is jetted through a restricted orifice in order to induce fine bubble formation in the main body of the beverage. This system has been commercialised, but it is widely accepted that jetting gas only rather than carbonated beverage or carbonated beverage followed by gas, provides better bubble nucleation and hence better head formation. GB-A-2,183,592 discloses a number of constructions wherein the secondary chamber may be constructed as an integral part of the beverage container or it may be formed as a discrete insert which is deposited or pushed into a conventional form of can, bottle or carton. Preference is expressed in GB-A-2,183,592 for an insert which is retained in position, for example at the bottom of the container, by an appropriate adhesive or by mechanical means. However, there is described the possibility of using a discrete insert which may be suspended or float in the beverage in the container provided that the restricted orifice is maintained below the surface of the beverage in the container on opening the container. The possibility of loading or weighting the insert to orientate the position of the restricted orifice is described.
EP-A-0,520,646 describes another proposal in which a beverage container has an insert with a restricted orifice which is arranged to jet gas only into the beverage. This insert is charged with gas by inverting the container promptly after it has been filled with beverage and the headspace above the beverage in the container pressurised so that the restricted orifice is exposed to pressure within the headspace above the beverage in the inverted container. Failure to ensure that the container remains inverted during the pressurization stages, including pasteurisation, results in the insert being filled with a significant amount of beverage, thereby losing all the benefits to be achieved by ejection of gas only under pressure from the insert when the container is opened. In practice, this can occur when there is an unforeseen production line stoppage which results in containers being stopped before inversion. Additionally, during pasteurisation, containers frequently fall over and are pasteurised on their side, in which orientation it is possible for substantial amounts of the beverage to enter the insert, especially since a high pressure exists in the container as a result of heating of the sealed container to the pasteurisation temperature.
With both the systems described in GB-A-2,183,592 and EP-A-0,520,646 since the insert is open via its restricted orifice before it is placed into the beverage container it is full of air. It is essential however to remove all of the air from the insert and container combination before filling it with beverage. The presence of oxygen inside the container leads to the beverage being oxidised with the resulting impairment of flavour and risk of microbial growth leading to, for example, acetification of the beverage when it contains alcohol. This removal of air is difficult to achieve in practice. Typically the container and insert combination is subjected to a purging regime using an inert gas such as nitrogen, carbon dioxide or a mixture of these and repeated pressurisation and depressurisation stages. This requires the use of an especially modified filling machine and substantially increases the filling cycle time.
This difficulty has been overcome in a system disclosed in WO-A-91/07326 in which an insert which jets gas only into the beverage in the main body of the container is pre-pressurized with gas and includes a closure means. The closure means remains sealed before filling and during the container filling operation but when the beverage container is subsequently opened, de-pressurisation of the beverage container results in the insert releasing a surge of gas from a restricted orifice into the beverage to "seed" the required nucleation of dissolved gas bubbles to produce the required rich creamy foam. This system has met with considerable commercial success. Since the insert is sealed at all material times before the container is finally opened by the consumer the container and insert combination can be filled as easily, simply and quickly as conventional container. A disadvantage of this type of system is that the insert may contain a residual pressure after the container has been emptied. There is a risk a consumer will cut open the empty container and thus be able to interfere with a pressurised insert.
WO-91/07326 discloses a very large number of ways in which the pressurized gas insert can be formed and mounted within the beverage container. In most examples, the insert is mounted so that, in use, it is located at a fixed position. However, an example is also described where the insert floats in the liquid in the container and has a weight attached to its base for orientating the insert so that the restricted orifice is submerged in the beverage.